I dont know if this is an error somewhere in the documentation, but we dont have reply nor chat icons for tabs.
Looking at the css these are the available tab iconCls options.

Code:

bookmarks
download
favorites
info
more
search
time
user
team
settings

The settings icon works for me on the button in the toolbar. Note that docking the button to the right of the toolbar is not the preferred solution. We only did this in the guide because we were adding it to a TabBar, and we didnt want to button to be part of the items (and thus tabs) collection.

P.s. You might want to add some spacing to make your code more readable. E.g.

There are two sets of icons. One is for buttons inside a toolbar, and the other is for tabs in a tabbar. The ones I listed are all the available icons specifically for tabs. We should probably make this more clear in the example and documentation.
I think the reason that they can not be shared is because tab icons are a different size then toolbar button icons. It do agree though that it would be nice if you could use every iconCls on both buttons and tabs.